continuous actuators - 24.23

Figure 24.20 Equations for a controlled field motor

24.3 HYDRAULICS

Hydraulic systems are used in applications requiring a large amount of force and slow speeds. When used for continuous actuation they are mainly used with position feedback. An example system is shown in Figure 24.21. The controller examines the position of the hydraulic system, and drivers a servo valve. This controls the flow of fluid to the actuator. The remainder of the provides the hydraulic power to drive the system.

continuous actuators - 24.24

sump

Figure 24.21 Hydraulic Servo System

The valve used in a hydraulic system is typically a solenoid controlled valve that is simply opened or closed. Newer, more expensive, valve designs use a scheme like pulse with modulation (PWM) which open/close the valve quickly to adjust the flow rate.

24.4 OTHER SYSTEMS

The continuous actuators discussed earlier in the chapter are the more common types. For the purposes of completeness additional actuators are listed and described briefly below.

Heaters - to control a heater with a continuous temperature a PWM scheme can be used to limit a DC voltage, or an SCR can be used to supply part of an AC waveform.

Pneumatics - air controlled systems can be used for positioning with suitable feedback. Velocities can also be controlled using fast acting valves.

Linear Motors - a linear motor works on the same principles as a normal rotary motor. The primary difference is that they have a limited travel and their cost is typically much higher than other linear actuators.

Ball Screws - rotation is converted to linear motion using balls screws. These are low friction screws that drive nuts filled with ball bearings. These are normally used with slides to bear mechanical loads.

continuous actuators - 24.25

24.5SUMMARY

•AC motors work at higher speeds

•DC motors work over a range of speeds

•Motion control introduces velocity and acceleration limits to servo control

•Hydraulics make positioning easy

24.6PRACTICE PROBLEMS

1.A stepping motor is to be used to drive each of the three linear axes of a cartesian coordinate robot. The motor output shaft will be connected to a screw thread with a screw pitch of 0.125”. It is desired that the control resolution of each of the axes be 0.025”

a)to achieve this control resolution how many step angles are required on the stepper motor?

b)What is the corresponding step angle?

c)Determine the pulse rate that will be required to drive a given joint at a velocity of 3.0”/sec.

2.For the stepper motor in the previous question, a pulse train is to be generated by the robot controller.

a)How many pulses are required to rotate the motor through three complete revolutions?

b)If it is desired to rotate the motor at a speed of 25 rev/min, what pulse rate must be generated by the robot controller?

3.Explain the differences between stepper motors, variable frequency induction motors and DC motors using tables.

continuous actuators - 24.26

24.7 PRACTICE PROBLEM SOLUTIONS

1.

2.

3.

24.8 ASSIGNMENT PROBLEMS

1.A stepper motor is to be used to actuate one joint of a robot arm in a light duty pick and place application. The step angle of the motor is 10 degrees. For each pulse received from the pulse train source the motor rotates through a distance of one step angle.

a)What is the resolution of the stepper motor?

b)Relate this value to the definitions of control resolution, spatial resolution, and accuracy, as discussed in class.

c)For the stepper motor, a pulse train is to be generated by a motion controller.

continuous actuators - 24.27

How many pulses are required to rotate the motor through three complete revolutions? If it is desired to rotate the motor at a speed of 25 rev/min, what pulse rate must be generated by the robot controller?

2.Describe the voltage ripple that would occur when using a permanent magnet DC motor as a tachometer. Hint: consider the use of the commutator to switch the polarity of the coil.

3.Compare the advantages/disadvantages of DC permanent magnet motors and AC induction motors.